This invention relates to a sensor for detecting acceleration and a method of producing such a sensor.
Japanese Laid-Open (Kokai) Patent Application No. 109374/88, filed by the Applicant of this application, discloses an acceleration sensor which comprises a tubular casing, a magnetic member in the form of an iron ball received within the casing, a differential transformer (detection member) mounted externally of the casing, and a ring-shaped permanent magnet disposed externally of the differential transformer to attract the iron ball to a central portion of the differential transformer. The casing, the differential transformer and the permanent magnet are mounted within a body immovably relative to the body. The sensor is mounted on an object to be sensed in such a manner that the axis of the casing coincides with the sensing direction.
In the above conventional acceleration sensor, particularly, the relation between the magnetic force of the permanent magnet and the position of the casing, as well as the relation between the magnetic force of the permanent magnet and the position of the differential transformer, is important. However, intended functions have not always been achieved due to manufacturing errors of the component parts and an error in magnetization of the permanent magnet.
Japanese Laid-Open Patent Application No. 144261/88 and Japanese Laid-Open Utility Model Application Nos. 97862/88, 109654/88 and 129866/88, filed by the Applicant of this application, also disclose acceleration sensors employing a tubular casing, an iron ball, a permanent magnet and a differential transformer. Based on the above five earlier Japanese applications and other applications, a U.S. Pat. application was filed on Oct. 23, 1987 under U.S. Pat. Ser. No. 113,180.
U.S. Pat. Nos. 2,979,959, 4,311,051 and 4,365,513 describe acceleration sensors using an iron ball.
Japanese Laid-Open Patent Application No. 62870/86 discloses a two-dimensional acceleration sensor comprising a pair of permanent magnets disposed respectively on opposite sides of a disc-shaped casing.
Further, U.S. Pat. Nos. 3,100,292 and 4,047,439 and Japanese Laid-Open Patent Application Nos. 203861/85, 233564/85 and 252271/85 disclose acceleration sensors.
However, none of the above-mentioned prior art publications disclose means for adjusting the relative position between the permanent magnet and the casing and the relative position between the permanent magnet and the differential transformer.
In the acceleration sensor disclosed in the above-mentioned Japanese Laid-Open Utility Model Application No. 129866/88 and the above-mentioned U.S. Pat. Ser. No. 113,180 based thereon, a damper liquid is filled in a sealed space provided within the casing. When the acceleration sensor is subjected to acceleration due to vibration, the damper liquid acts on the iron ball and prevents the iron ball from being subjected to resonance. However, some problems have been encountered with this conventional sensor. More specifically, the sealed space within the casing is defined by the casing body with one open end and a rigid lid threadedly connected to the casing body to close the open end of the casing body. Therefore, the thermal expansion of the damper liquid developing with a temperature increase is prevented by the casing body and the lid. Therefore, the pressure within the sealed space increases, and as a result the casing body is subjected to distortion. Particularly when such distortion develops in the casing body in the axial direction thereof, the relative position between the differential transformer and the magnet is changed, so that the relation between the output of the differential transformer and the acceleration is changed, thus failing to achieve a precise detection of the acceleration.
Further, in the acceleration sensor of such a construction, a small amount of the air is inevitably present in the damper liquid within the sealed space. When such small amount of the air moves within the sealed space, the position of the iron ball is slightly changed when the acceleration is either zero or very small, so that the output of the differential transformer is not kept to zero. As a result, the acceleration can not be detected with high precision when the acceleration is either zero or very small. The introduction of the air into the damper liquid is unavoidable with the acceleration sensor of such a construction. The reason is that if the damper liquid is fully filled in the casing body, the lid can not be properly threaded into the open end portion of the casing body. Therefore, the damper liquid is supplied into the casing body to a predetermined liquid level to enable such threaded connection, and in this case even at an initial stage of the threading of the lid into the casing body, the air is entrapped in a space between the lid and the liquid surface of the damper liquid.